The present invention relates to interferometers, and more particularly to a heterodyne interferometer of improved construction and performance.
Vital to the construction of a heterodyne interferometer is the generation of the frequency or phase shift of one leg with respect to the other in the test and reference legs of the interferometer. The usefulness of such a device is somewhat offset by the difficulty of the critical alignment of the optics associated with producing a beam composed of the two orthogonally polarized components, one of which is frequency shifted with respect to the other.
One of the more common frequency (phase) shifting techniques includes acousto-optic modulators (AOMs) which are similar in principle of operation to moving gratings and Bragg cells. They deflect a light beam at normal incidence to an acoustic sound field at an angle that is proportional to the acoustic frequency. The diffracted light is Doppler-shifted (up or down) by the frequency of the acoustic field. In a generic heterodyne frequency generation configuration, an input beam is split into separate orthogonal polarizations and directed into separate AOMs. One modulator is driven at its center frequency while the other is operating at the center frequency plus some offset. The first diffraction order beams are then recombined and carefully aligned to be colinear. The heterodyne frequency generated is equal to the frequency offset between the two AOMs. The main disadvantage to this technique of generating the heterodyne frequency is the complex alignment techniques associated with splitting, modulating and then recombining the beams from the two AOMs.
Utilizing a rotating halfwave plate as a frequency shifter has been discussed by R. Crane in an article entitled "Interference Phase Measurement", Applied Optics, March 1969, and also by G. Sommargren in an article entitled "Up/down Frequency Shifter for Optical Heterodyne Interferometry", Journal of the Optical Society of America, August 1975. While there are subtle differences between the two configurations, both rely on a rotating halfwave plate to provide a continuous phase shift which results in a translation in frequency equal to twice the rotational frequency. The mechanical action associated with the plate rotation limits the optical frequency shift to about two kilohertz and can create vibrational problems.
It is also known that by displacing a mirror at a constant rate, it is possible to Doppler shift the light reflecting off of the mirror surface. One possible implementation of this concept would involve a mirror mounted on the surface of a piezoelectric translator (PZT) which is driven by a ramp wave. It is not possible to achieve continuous modulation since the PZT would have to be reset at the end of each ramp cycle. The heterodyne frequency would also be limited by the velocity of the mirror as driven by the PZT translator. The mirror could also be dithered sinusoidally but the resulting modulation sidebands would consist of first order Bessel functions, and a pure frequency shift could not be realized.
There have also been devices designed to utilize electrooptic cells, such as Pockels cells or Kerr cells, as phase/frequency modulators. The optical activity of such cells is varied by means of electric fields applied thereto. While many commercial electrooptic cells have very high bandwidths, some in the hundreds of megaHertz, they also have halfwave voltages in the kilovolt range. Thus the performance of such electrooptic cells is limited by the capabilities of the driving source. Pockels cells also have small spatial apertures and collimation requirements that are typically within one degree. If a Pockels cell electrooptic modulator is driven with a sinusoidal signal, the resulting phase modulation will generate unwanted sidebands as in the case of the dithering mirror. An interferometer which uses a pair of aligned Pockels cells or Kerr cells is disclosed in U.S. Pat. No. 4,180,328 issued to L. E. Drain on Dec. 25, 1979.